A known fuel pump comprises a substantially disk-shaped impeller and a casing. A pair of grooves is formed in a pair of interior surfaces of the casing and each groove extends continuously in a circumference direction from an up-stream end to a down-stream end. A suction hole is formed to pass through the casing for communicating the exterior of the casing to the up-stream end of the circumferential groove, and a discharge hole is formed to pass through the casing for communicating the down-stream end of the circumferential groove to the exterior of the casing. When the impeller is rotated within the casing, a fuel is sucked into the casing, the pressure thereof is increased within the casing and the pressurized fuel is discharged from the casing.
The cross-sectional area of the circumferential groove formed in the interior surface of the casing greatly affects the efficiency of the fuel pump. The fuel pump disclosed in Japanese Laid-Open Patent Publication 7-189975 divides the circumferential groove into a first region which is cross to the up-stream end and a second region which is cross to the down-stream end. In the first region, the cross-sectional area of the circumferential groove becomes smaller toward the down-stream side, while the cross-sectional area of the groove is constant in the second region.